1. Field of the Invention
This invention relates to a light measurement compensation device in the TTL light measuring type camera capable of changing over the distribution of sensitivity of the light measurement.
2. Description of the Prior Art
In general, the amount of light received by the photosensitive element in the TTL light measuring type camera is assumed to be proportional to the area of aperture opening of the diaphragm of the photographic lens. In a strict sense, however, there are cases where this proportional relationship does not hold. The first of these causes appears when the lens of a small F-number at full open aperture is used with the aperture full opened or thereabout (hereinafter called case A). The second cause appears when the partial light measuring mode is operated with a large F-number, that is, the aperture closed down (hereinafter called case B).
Here, the foregoing is further explained in more detail. FIG. 1 is a graph illustrating the relationship between the area of aperture opening of the photographic lens and the output of the light measuring circuit with a solid line curve A for the average light measurement and a dashed line curve B for partial light measurement. It is in the region of F/1.4, as the full open aperture, to F/2.8 that the light response characteristic becomes non-linear due to the diffusing property of the focusing screen glass plate, and therefore the proportional relationship is broken in the aforesaid case A. In other words, because the photosensitive element is arranged behind the focusing screen at either side of the eyepiece lens, or in the neighborhood of the penta prism, and because the focusing screen is not of perfect diffusing surface for the purpose of increasing the brightness of the finder image, as the size of the aperture opening of the lens approaches the full open F-number, the ratio of the straight going component to the diffusing component of the light incident on the photosensitive element varies. Thus, the output characteristic of the light measuring circuit becomes non-linear, and the proportional relationship does not hold. In this case, with the use of a faster lens than F/2.8 at the full open aperture, when the light measuring is performed with the full open aperture, an over-exposure will result throughout the entire range of adjustment of the size of aperture opening. For example, when F/1.4, or F/2.0, an exposure error corresponding to .theta.1 or .theta.2 (see FIG. 1) respectively is produced.
The correction for this exposure error, called curvature correction, is introduced into a system for processing the output of the light measuring circuit by an aperture correction pin on the lens mounting and a variable resistor in the camera body so that the apparent output characteristic of the light measuring circuit in the full open aperture-average light measuring and the full open aperture-partial light measuring modes becomes substantially equivalent to the output characteristic of the light measuring circuit when in the stop-down partial light measuring mode as indicated by a line AA in FIG. 1.
In the remaining region of F/2.8 to F/16, on the other hand, a gradient difference arises in the output characteristic of the light measuring circuit between the average and partial light measuring modes (the aforesaid case B). This is attributable to the arrangment of the light measuring optical system. Here, the outline of the light measuring optical system is illustrated in FIG. 2, where 301 is a photographic lens; 302 is a focusing screen having a split image prism 303; 304 is an eyepiece lens; 104 is a collection lens for the photosensitive element 31. The details of the light receiving surface of the element 31 is shown in FIG. 3, wherein as the photosensitive element 31 and the focusing screen 302 are conjugate to each other, when in the partial light measuring mode, the central area 93 only, or when in the average light measuring mode, the central area 93 and the surrounding area 91 in combination, is or are electrically selected. Since the light beam passing through the split image prism 303 on the focusing screen 302 does not diffuse, the smaller the size of aperture opening of the lens, the weaker the intensity of light incident on the photosensitive element 31 becomes. This is the same phenomenon as that the split image prism 303 casts shadow in the field of view of the finder when the diaphragm of the lens is stopped down. Because this phenomenon becomes more prominent as the proportion of the image of the split image prism 303 to the light receiving surface increases, for the average light measuring mode it has little influence, but for the partial light measuring mode, the influence creates a serious problem. The difference between the gradients of variation of the light value for the average and partial light measuring modes seen in FIG. 1 is based on such reason.
In the camera operating with selection of many light measuring sensitivity patterns such as the average and spot light measuring modes, therefore, when the camera was changed over between two of the stop down-average light measuring, stop down-spot light measuring, full open aperture-average light measuring and full open aperture-spot light measuring modes, there was produced as much a light value difference as not to be possible to compensate for by the prior known curvature correction technique alone. In connection with the example of FIG. 1, it has been found that when in the stop down-spot light measuring mode, the error is largest among the other modes, reaching 0.7 EV for the aperture value of F/11, and 0.8 EV for F/16 by which the actual exposure is more excessive than the correct one. With the use of a digital exposure display in 0.5 EV increments, as the light value difference takes more than 0.5 EV between the light measuring modes, there is a high possibility of production of a difference of 1.0 EV on the display. This leads to a problem of lowering the reliability and accuracy of exposure control.